The invention relates generally to implanted or implantable blood pump systems, and more specifically, to a method and system for physiologic control of such pumps. 2. DESCRIPTION OF RELATED ART
Generally, implantable blood pump systems are employed in either of two circumstances. First an implantable blood pump may completely replace a human heart that is not functioning properly, or second, an implantable blood pump may boost blood circulation in patients whose heart is still functioning although pumping at an inadequate rate.
For example, U.S. Pat. No. 6,183,412, which is commonly assigned and incorporated herein by reference in its entirety, discloses a ventricle assist device (VAD) commercially referred to as the “DeBakey VAD®.” The VAD is a miniaturized continuous axial-flow pump designed to provide additional blood flow to patients who suffer from heart disease. The device is attached between the apex of the left ventricle and the aorta.
Known implantable blood pump systems typically are controlled in an open loop fashion where a predetermined speed is set and the flow rate varies according to the pressure differential across the pump. The pump itself may be controlled in a closed loop fashion, wherein the actual pump speed is fed back to a motor controller that compares the actual speed to the desired predetermined speed and adjusts the pump accordingly. However, prior art closed loop control systems—varying the pump speed in response to a monitored physiologic parameter—have largely been unsatisfactory.
For example, some systems have attempted to use a patent's heart rate, or pulse, as a physiologic control trigger—the pump speed set point is varied in response to the patient's heart rate . Other systems attempt to vary the pump speed based on the variation of the VAD pump's flow or current signals with respect to the signal's mean value or with respect to pump speed. For example, a “pulsatility index” is derived
  (      e    .    g    .                  ⁢                            Signal                      MA            ⁢                                                  ⁢            X                          -                  Signal                      MI            ⁢                                                  ⁢            N                                      Signal        MEAN              )and compared to a predetermined threshold and the pump speed is varied accordingly.
Unfortunately, these physiologic control methods have not provided an adequate closed loop control parameter, as it appears that known physiologic control parameters such as these do not necessarily vary proportionally to a patient's level of activity—i.e., the patient's demand for increased blood flow. Further, while a patient's heart rate may increase during exercise, heart rate may be controlled by other factors, such as medication or a pacing device. Still further, the patient may not have significant native heart rate function, preventing the heart rate from increasing in response to the body's demand for increased blood flow. Moreover, there exists some evidence that a patient's heart rate may decrease as the pump's speed is increased. Hence, heart rate alone may not provide a satisfactory physiologic control parameter.
The present invention addresses shortcomings associated with the prior art.